Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Baeldung)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat=Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag=Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
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Let's get started with a Microservice Architecture with Spring Cloud:

>> Join Pro and download the eBook

eBook – Mockito – NPI EA (tag = Mockito)
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Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

Get started with mocking and improve your application tests using our Mockito guide:

Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Reactive – NPI EA (cat=Reactive)
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Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

>> Join Pro and download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Jackson – NPI EA (cat=Jackson)
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Do JSON right with Jackson

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eBook – HTTP Client – NPI EA (cat=Http Client-Side)
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Get the most out of the Apache HTTP Client

Download the E-book

eBook – Maven – NPI EA (cat = Maven)
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Get Started with Apache Maven:

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eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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eBook – RwS – NPI EA (cat=Spring MVC)
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Building a REST API with Spring?

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Course – LS – NPI EA (cat=Jackson)
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Get started with Spring and Spring Boot, through the Learn Spring course:

>> LEARN SPRING
Course – RWSB – NPI EA (cat=REST)
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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

>> The New “REST With Spring Boot”

Course – LSS – NPI EA (cat=Spring Security)
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Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

You can explore the course here:

>> Learn Spring Security

Course – All Access – NPI EA (cat= Spring)
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All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

>> GET THE COURSE
Course – LSD – NPI EA (tag=Spring Data JPA)
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Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

Get started with Spring Data JPA through the guided reference course:

>> CHECK OUT THE COURSE

Partner – LambdaTest – NPI EA (cat=Testing)
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End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (cat=Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

eBook – Reactive – NPI(cat= Reactive)
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Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

>> Join Pro and download the eBook

eBook – HTTP Client – NPI (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

>> Download the eBook

1. Overview

Typically when making HTTP requests in our applications, we execute these calls sequentially. However, there are occasions when we might want to perform these requests simultaneously.

For example, we may want to do this when retrieving data from multiple sources or when we simply want to try giving our application a performance boost.

In this quick tutorial, we’ll take a look at several approaches to see how we can accomplish this by making parallel service calls using the Spring reactive WebClient.

2. Recap on Reactive Programming

To quickly recap WebClient was introduced in Spring 5 and is included as part of the Spring Web Reactive module. It provides a reactive, non-blocking interface for sending HTTP requests.

For an in-depth guide to reactive programming with WebFlux, check out our excellent Guide to Spring 5 WebFlux.

3. A Simple User Service

We’re going to be using a simple User API in our examples. This API has a GET method that exposes one method getUser for retrieving a user using the id as a parameter.

Let’s take a look at how to make a single call to retrieve a user for a given id:

WebClient webClient = WebClient.create("http://localhost:8080");
public Mono<User> getUser(int id) {
    LOG.info(String.format("Calling getUser(%d)", id));

    return webClient.get()
        .uri("/user/{id}", id)
        .retrieve()
        .bodyToMono(User.class);
}

In the next section, we’ll learn how we can call this method concurrently.

4. Making Simultaneous WebClient Calls

In this section, we’re going see several examples for calling our getUser method concurrently. We’ll also take a look at both publisher implementations Flux and Mono in the examples as well.

4.1. Multiple Calls to the Same Service

Let’s now imagine that we want to fetch data about five users simultaneously and return the result as a list of users:

public Flux fetchUsers(List userIds) {
    return Flux.fromIterable(userIds)
        .flatMap(this::getUser);
}

Let’s decompose the steps to understand what we’ve done:

We begin by creating a Flux from our list of userIds using the static fromIterable method.

Next, we invoke flatMap to run the getUser method we created previously. This reactive operator has a concurrency level of 256 by default, meaning it executes at most 256 getUser calls simultaneously. This number is configurable via method parameter using an overloaded version of flatMap.

It’s worth noting, that since operations are happening in parallel, we don’t know the resulting order. If we need to maintain the input order, we can use flatMapSequential operator instead.

As Spring WebClient uses a non-blocking HTTP client under the hood, there is no need to define any Scheduler by the user. WebClient takes care of scheduling calls and publishing their results on appropriate threads internally, without blocking.

4.2. Multiple Calls to Different Services Returning the Same Type

Let’s now take a look at how we can call multiple services simultaneously.

In this example, we’re going to create another endpoint which returns the same User type:

public Mono<User> getOtherUser(int id) {
    return webClient.get()
        .uri("/otheruser/{id}", id)
        .retrieve()
        .bodyToMono(User.class);
}

Now, the method to perform two or more calls in parallel becomes:

public Flux fetchUserAndOtherUser(int id) {
    return Flux.merge(getUser(id), getOtherUser(id));
}

The main difference in this example is that we’ve used the static method merge instead of the fromIterable method. Using the merge method, we can combine two or more Fluxes into one result.

4.3. Multiple Calls to Different Services Different Types

The probability of having two services returning the same thing is rather low. More typically we’ll have another service providing a different response type and our goal is to merge two (or more) responses.

The Mono class provides the static zip method which lets us combine two or more results:

public Mono fetchUserAndItem(int userId, int itemId) {
    Mono user = getUser(userId);
    Mono item = getItem(itemId);

    return Mono.zip(user, item, UserWithItem::new);
}

The zip method combines the given user and item Monos into a new Mono with the type UserWithItem. This is a simple POJO object which wraps a user and item.

5. Testing

In this section, we’re going to see how we can test the code we’ve already seen and, in particular, verify that service calls are happening in parallel.

For this, we’re going to use Wiremock to create a mock server and we’ll test the fetchUsers method:

@Test
public void givenClient_whenFetchingUsers_thenExecutionTimeIsLessThanDouble() {
        
    int requestsNumber = 5;
    int singleRequestTime = 1000;

    for (int i = 1; i <= requestsNumber; i++) {
        stubFor(get(urlEqualTo("/user/" + i)).willReturn(aResponse().withFixedDelay(singleRequestTime)
            .withStatus(200)
            .withHeader("Content-Type", "application/json")
            .withBody(String.format("{ \"id\": %d }", i))));
    }

    List<Integer> userIds = IntStream.rangeClosed(1, requestsNumber)
        .boxed()
        .collect(Collectors.toList());

    Client client = new Client("http://localhost:8089");

    long start = System.currentTimeMillis();
    List<User> users = client.fetchUsers(userIds).collectList().block();
    long end = System.currentTimeMillis();

    long totalExecutionTime = end - start;

    assertEquals("Unexpected number of users", requestsNumber, users.size());
    assertTrue("Execution time is too big", 2 * singleRequestTime > totalExecutionTime);
}

In this example, the approach we’ve taken is to mock the user service and make it respond to any request in one second. Now if we make five calls using our WebClient we can assume that it shouldn’t take more than two seconds as the calls happen concurrently.

To learn about other techniques for testing WebClient check out our guide to Mocking a WebClient in Spring.

6. Conclusion

In this tutorial, we’ve explored a few ways we can make HTTP service calls simultaneously using the Spring 5 Reactive WebClient.

First, we showed how to make calls in parallel to the same service. Later, we saw an example of how to call two services returning different types. Then, we showed how we can test this code using a mock server.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
announcement - icon

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
announcement - icon

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
announcement - icon

The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

>> Download the eBook

eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
announcement - icon

Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

Explore the eBook

Course – LS – NPI EA (cat=REST)

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Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – Spring Sale 2025 – NPI (All)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

>> EXPLORE ACCESS NOW

Course – LS – NPI (cat=HTTP Client-Side)
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Get started with Spring Boot and with core Spring, through the Learn Spring course:

>> CHECK OUT THE COURSE

Partner – Microsoft – NPI (cat=Spring)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

eBook Jackson – NPI EA – 3 (cat = Jackson)